Provides metrics related to scan operations (both server side and client side metrics).

The data can be passed to mapreduce framework or other systems.
We use atomic longs so that one thread can increment,
while another atomically resets to zero after the values are reported
to hadoop's counters.

Some of these metrics are general for any client operation such as put
However, there is no need for this. So they are defined under scan operation
for now.